1. Field of the Invention
The invention relates in general to electrical connections, and more specifically to electrical connections of a plurality of electrical conductors to an electrical bus bar.
2. Description of the Prior Art
The individual coils of an electrical power transformer have a plurality of conductor turns, with the conductor of the coil including a plurality of assembled straps or conductors which have a generally rectangular cross-sectional configuration. The coils are interconnected to provide a transformer winding, and the ends of the winding are connected to electrical bus bars. The connection to the bus bar must interconnect the ends of the individual straps to one another, and to the bus bar, with a joint which is good mechanically as well as electrically. To avoid a high resistance region in the current path, the cross-sectional area of the connection must be at least equal to the cross-sectional area of the assembled coil conductor.
Connections made in high current transformers having copper conductors and bus bars are usually made by brazing the conductors directly to the bus bar. Copper brazing is performed by heating the straps and the bus bar terminal either with the incandescent or the gas torch method. A self-fluxing filler material, such as phos-copper or phos-silver is fed to the joint for solidifying the line leads to the bus bar terminal. Since the brazing temperature is in the range of 1202.degree. to 1508.degree. F., and the melting point for copper is 1,980.degree. F., special temperature control is not required. This method forms an excellent connection because the brazing material readily flows between the conductor straps as well as between the bottom straps and the bus bar. The connection is completely solid.
Connections made in high current power transformers having aluminum conductors and bus bars are more complicated to make than connections in copper transformers. Brazing aluminum requires precise temperature control since the melting point of aluminum is only 100.degree. to 150.degree. F. above the melting point of the brazing material. The allowable tolerance of the temperature is too low to permit satisfactory results in a production shop.
Brazing aluminum also requires the use of chemical flux materials to promote wetting action. Unfortunately, suitable fluxes are corrosive to the aluminum, with any residual flux trapped in the joint causing corrosion of the connection with degrades its electrical performance.
It has been found that arc welding by either the MIG (Metallic Inert Gas) or TIG (Tungsten Inert Gas) method provides a satisfactory electrical and mechanical connection for the aluminum members. When the number of conductor straps and the size of the conductor are sufficiently small, arc welding may be used to weld the ends and the edges of the aluminum straps directly to an aluminum bus bar. However, when the size of the conductor increases due to the number and/or size increase of the straps, the arc welding method of connecting the straps directly to the bus bar becomes impractical. This is due to low capillary action in aluminum welding, resulting in an extremely low build of weld material during each pass. As a general rule, the weld material should have a current carrying cross-sectional area equal to or greater than the cross-sectional area of the conductor. To achieve this, it is necessary to build up the weld material by making a plurality of passes with the welding electrode. The large number of passes required for aluminum welding the number and size of the conductor straps in a typical high current power transformer coil conductor to a bus bar makes the coil lead termination difficult, time consuming, and therefore very costly.
U.S. Pat. No. 3,742,122, which is assigned to the same assignee as the present application, discloses a new arrangement for making good mechanical and electrical connections between the straps of a multiple aluminum conductor, and an aluminum bus bar. This new and improved arrangement does not require an excessive weld buildup to achieve good electrical performance, regardless of the number and size of the assembled straps which make up the coil conductor. This new arrangement connects at least two aluminum partitions to the aluminum bus bar to define a channel, or a U-shaped fixture having upstanding partitions is fastened to the bus bar, which fixture defines a channel, and the assembled strap conductors are positioned in the channel with their ends substantially aligned with selected flat surfaces of the partitions. A fusion of metal, such as an alloy of aluminum, is applied across the aligned conductor ends and adjoining flat surfaces of the partitions by MIG or TIG welding.
The connecting arrangement in the hereinbefore mentioned U.S. patent is suitable for a large coil conductor having a large number of assembled strap conductors, but the U-shaped connector with the partitions and connecting base portion produces an objectionally bulky terminal for terminating smaller coil conductors, which conductors have fewer and smaller conductor straps. The U-shaped connector also must be premade with accurate channel widths, and since different transformer ratings utilize different numbers and sizes of strap conductors in the coil conductor, it is necessary to manufacture a relatively large number of different sizes of U-shaped connectors.
If the partitions are fastened directly to the bus bar, the resulting connection is still bulky for the smaller coil conductors, and, the welding of the partitions to the bus bar requires a high degree of skill to provide a channel which enables the coil conductor to snugly fit the channel without interference with fillet weld beads.